Best Open Source Database for Data Warehouse

We are in the process of selecting an Open Source database for Data Warehouse for a BI Project (Open Source BI). We are toying up with:
1. MySQL
2. PostgreSQL
The daily updation of data (in operational database.....mainly log files)would be in range of 2-3 GBs per day with 5-6 millions rows of data added daily. We would like to know:
1. Experiences of people in DW using open source dbases (especially MySQL). Experiences on scaling, optimization, performance, storage.....what are the limitations?
2. Things to watch out for....pifalls?
3. Any other good open source dbase option than above
Thanks in advance for the help.